1. Field of the Invention
The present invention relates generally to calibration standards for electrodes. Particularly, the present invention relates to gas control liquids for quality control and/or calibration of gas analyzer equipment such as blood gas analyzers and water gas analyzers.
2. Description of the Prior Art
Various fluid analysis devices are used in testing and measuring gas and/or electrolyte levels in fluids. These devices are used, for example, in medical and environmental applications to measure the gas and/or electrolyte levels in blood, urine, water, and other fluids.
Fluid analysis devices must be calibrated frequently when used to test fluid samples. It is common practice to utilize control solutions for verifying the accuracy and reliability of these analysis devices. For example, blood gas analyzers typically include electrodes that measure pH, the partial pressure of carbon dioxide (pCO2) and the partial pressure of oxygen (pO2) of a blood sample. Such electrodes are typically calibrated prior to use for measuring a blood sample.
Calibration of an electrode involves contacting the electrode with standard solutions or gases having known concentrations of the substance being analyzed. The electrode provides an electrical response that is used to generate a calibration slope. The electrode then is contacted with the sample to be measured, which generates a further electrical response. The calibration slope is used to convert the electrical response into the concentration of the substance in the sample. For example, electrodes in blood gas analyzers are calibrated periodically because the response provided by an electrode for a particular sample tends to drift (i.e. vary) with the passing of time.
Typically, a reference or calibration container holds a reference or calibration solution that contains gases, such as oxygen and carbon dioxide, in solution at known partial pressures. Because these partial pressures are known to a relatively precise degree, the reference or calibration solution can be used to accurately calibrate the fluid analysis machine after fluid samples have been tested.
Gas tonometered aqueous solutions packaged in sealed containers have been used as calibration standards for oxygen and carbon dioxide electrodes. The manufacturing environment for the gas tonometered solutions requires precise temperature and pressure control to ensure the accuracy of the calibration standards. It is also necessary to equilibrate these tonometered solutions to a known temperature before using them to calibrate the electrodes in the analyzer.
Other liquid gas control products have been proposed which are based on components of human blood or components proposed for use as blood substitutes such as fluorocarbon and silicone compound emulsions.
Some control/calibration solutions have been provided in gas-tight, sealed ampuls and contain known concentrations of dissolved oxygen and carbon dioxide.
Analyzers such as, for example, blood gas analyzers use various methods of packaging calibration fluids so that dissolved gas levels are stable over a period of time; typically, the shelf life of these calibration fluids are eighteen (18) months or more. Bag/container materials and fitments for accessing the contents of the bag/container are selected to minimize gas diffusion either into or out of the bag/container depending on various factors such as atmospheric pressure and temperature.